Michael Banks on sun 4 apr 99
I think it is unlikely that magnesia or calcia from your glaze are
catalysing cristobalite in your ware. Hamer is only partly correct with
regard to catalysis of cristobalite. It is true that the presence of
cristobalite seems to trigger fine-grained quartz to recrystallize as the
cristobalite form. But the main mechanism for cristobalite production in
the presence of Ca & Mg are solid state reactions involving talc, whiting,
anorthite (Ca feldspar) and other Ca & Mg minerals. Talc in particular
produces cristobalite by dissociation above 900 degrees C. Other reactions
between CaO (from whiting) or MgO (from magnesium carbonate, dolomite etc)
with the aluminosilicate minerals (clays generally) also produce
cristobalite. The reason that soda and potash feldspars prevent this is
because they "mop up" the cristobalite as it is formed by fusing with it.
As for your dunting, it is much more likely that something has happened to
the clay in the factory. There are a great variety of 'accidents' that can
befall clay during the blending and blunging processes. For example filter
press cakes removed from one end of the press can be deficient in some of
the key ingredients if the slip feed has been too thin to keep all the
components in suspension.
It is possible that your body has become weakened to such an extent that it
cannot withstand the stresses placed on it by the normal inversion forces,
glaze compression/tension forces etc that you normally dish out to your
pottery. One of the worst problems to arise from filter-press gravity
segregation can arise from an over-concentration of fine-grained silica.
Many plastic ball clays have a fine silica component which can be
mechanically separated out by processes such as bad filter pressing. An
overabundance of this fraction in your pottery would not be detectable
during throwing/handbuilding but shows up during firing by symptoms similar
to those you describe.
Try swapping your batch of clay with another one made during a different
period. You may have heard of "Monday Cars", well Monday Clay occurs too!
John Hesselberth wrote:
After 25 years of making pots, I thought I had found all the
problems--well.....(clip) , --every large pot cracked. ....
Then I read on and noted that both calcium and magnesium are catalysts
for the conversion of silica to cristobalite.....